Installing NVIDIA drivers on Debian Lenny requires a little extra work compared to doing so in other distributions like Ubuntu, due to the lack of some required packages in the default installation. More here
Go to the NVIDIA official web site. Navigate to the drivers section. Choose the latest appropriate driver depending on your Linux platform (32- or 64-bit) and your graphic card model. Download the driver to your /home/username directory (or any other place of your choice).